Industrial electrolytic processes, for example the electrolysis of alkali brines, in particular of sodium chloride brine aimed at the production of chlorine, caustic soda and hydrogen, are commonly carried out in electrolysers consisting of a multiplicity of electrolytic cells divided by a separator, for example an ion-exchange membrane, into two compartments, anodic and cathodic, each containing an electrode.
The basic design commonly utilised provides that the anode compartment contains a rigid anode generally consisting of a punched plate or expanded sheet or metal mesh coated with a superficial electrocatalytic film comprising noble metal oxides. The structure of the cathode compartment may provide different types of mechanical arrangement. More precisely, the installation of the cathodes in the cathode compartment can be made according to two basic mechanical designs. A first design provides the cathode in direct contact with the membrane (design known among those skilled in the art as “zero-gap”), a second design provides the cathode to be spaced away from the membrane with gaps of 1-3 mm (design known among those skilled in the art as “finite gap”). In this second type of technology, the need to maintain a certain distance between the anodic and cathodic surfaces, approximately 2-3 mm, entails the cell voltage to be penalised by a component associated with the ohmic drop generated by the current transport in the liquid phase between the cathode and the membrane: since the cell voltage is directly proportional to the energy consumption, normally expressed in kWh per tonne of chlorine or caustic soda, it follows that the overall economy of the process is disadvantaged. To overcome this problem, the design of membrane electrolytic cells, especially for chlor-alkali electrolysis, has undergone major changes in time that gave rise to cathodic structures capable of bringing the surface of the cathode in contact with the membrane, a result designated by the aforementioned definition of “zero-gap”. In view of the increasingly high cost of energy and the unfavourable economy and feasibility entailed by a complete removal and replacement of “finite-gap” cells, it has been evidenced the need for a technology allowing to convert such cells present in electrolytic plants to the more efficient “zero-gap” technology taking advantage of the existing cell design and materials.